Copolymers of ethylene with one or more .alpha.-olefins and optionally a minor amount of polyenes are known which, after vulcanization exhibit elastomeric type properties. The most common of these are copolymers of ethylene with propylene (EPR), which can contain small amounts of a non-conjugated diene monomer (EPDM).
Thanks to their good resistance to high temperatures, chemical and atmospheric agents, and their dielectric properties, these elastomers find uses in various application fields. Principally they can be used as additives for lubricating oils, covering materials in building, electric insulators, components in the automotive industry or modifiers in mixtures with thermoplastic polymers.
The above mentioned ethylene copolymers are generally prepared in the presence of Ziegler-Natta catalysts. The copolymers so obtained have a high tendency to produce blocks of propylenic units in the chain, and this gives rise to crystallinity negatively influencing the final characteristic of the product. Therefore, in order to obtain vulcanized copolymers which are endowed with good elastomeric properties, it is important that the distribution of the propylenic units within the chain is as homogenous as possible.
More recently, these ethylene copolymers have been prepared in the presence of homogeneous catalysts obtained from metallocenes and aluminoxane compounds.
U.S. Pat. No. 5,001,205, for example, describes a process for the preparation of elastomeric copolymers of ethylene with an .alpha.-olefin, particularly with propylene, in which the product obtained from the reaction of methylaluminoxane (MAO) with a bis cyclopentadiene of Zr, Ti, or Hf, such as bis(tetrahydroindenyl)zirconium dichloride, ethylene-bis(tetrahydroindenyl)zirconium dichloride, or dimethylsilandiyl-bis(tetrahydroindenyl)zirconium dichloride is used as a catalyst.
The homogeneity of the distribution of the propylenic units in the copolymer that is obtained operating in the presence of these types of catalyst, although turning out improved with respect to the copolymers obtained from Ziegler-Natta catalysts, is still not totally satisfactory.